In combustion devices using liquid fuels, such as kerosene, the air for combustion should be supplied at such a rate as to provide a constant air-fuel ratio equivalent to the stoichiometric ratio. However, it has heretofore been very difficult to supply air at a predetermined rate in a stabilized manner because of the influences of the wind and the like. This results in too little or too much air being supplied, degrading the combustion state to the extent of increasing the amount of toxic substances in the exhaust gases, lowering the heat efficiency and, in extreme cases, making it impossible for the combustion to continue.
Such problems, which are due to variations in the air-fuel ratio, also occur when the fuel supply rate is changed. More particularly, the adjustment of the combustion state is effected by adjusting the atomization rate in the gun type, the injection rate in the pot type and the fuel vaporization area in the wick type, all using liquid fuels, and in the case of the types using gas fuels, it is effected by adjusting the gas flow rate. In either case, the rate of fuel supply to the combustion section is adjusted. If, therefore, the air supply rate is constant, there is a danger of producing variations in the air-fuel ratio, thus causing said problems. As a result, the range in which the combustion rate can be adjusted is limited by the air-fuel ratio and this range cannot be widened without sacrificing the optimum air-fuel ratio to some extent, which would aggravate the combustion state. Further, even if the air-fuel ratio is within said range, it can deviate from the optimum value, in which case it is liable to be influenced by the wind.
Since the conventional adjustment of the combustion rate is effected by directly increasing or decreasing the fuel supply rate, the adjusting device, particularly for combustion devices using liquid fuels, inevitably comes in contact with the liquid fuel and eventually malfunctions as it is corroded by the water present in the liquid fuel, by organic acids produced by degradation of the liquid fuel and/or by tarry materials adhering thereto, and the maintenance and inspection thereof have been difficult.
There is also a known combustion device of the type in which the fuel and air supply rates are correlatively adjusted by mechanical damper control means. However, the construction is complicated and, moreover, as the air flow rate decreases, the range becomes narrower in which the rate of supply of fuel required for maintaining the air-fuel ratio at the proper value is allowed to vary. In combustion devices using said mechanical opening control means, therefore, there have been problems, such as high accuracy being required in designing and producing the device.